A typical conventional spark plug comprises an electrically-insulating sleeve which extends along a central axis of the plug. Such sleeves are made of ceramic material, usually alumina. The plug also comprises a first electrode mounted within the sleeve and having a tip projecting beyond the sleeve. The electrode extends centrally within the sleeve and is electrically connected to a terminal projecting from the other end of the sleeve. The connection between the terminal and the first electrode includes a resistor also contained within the sleeve which serves to control to peak current. In the operation of the plug, a high tension lead is applied to the terminal so that a high voltage can be applied to the first electrode. The plug also comprises an electrically-conducting shell surrounding such sleeve. The shell is fixed, normally by a screw thread, into the head of an engine so that the tip of the first electrode projects into the combustion chamber of a cylinder of the engine. The plug also comprises a second electrode mounted on the shell, normally by welding, and electrically-connected to the shell. The second electrode has a tip which is positioned within the combustion chamber in opposed-relationship to the tip of the first electrode.
In the typical conventional spark plug described above, the tips of the electrodes each have a spark surface facing the spark surface of the other electrode so that the spark surfaces of the two electrodes define a spark gap of the plug. When a high voltage is applied to the first electrode, a spark jumps the spark gap and goes to ground through the second electrode, the shell, and the engine head. As it jumps the gap, the spark ignites fuel in the combustion chamber. The spark surface of the first electrode is arranged to extend in a plane which is normal to the central axis of the plug and the spark surface of the second electrode extends parallel to the spark surface of the first electrode so that the spark gap has a constant width along its length.
It has previously been proposed that the spark surfaces of the first and second electrodes should be inclined at the same angle relative to the plane extending normally of the central axis of the plug. This increases the surface area of the spark surfaces, thereby reducing the effects of wear and deposit build-up. It has also been proposed (see GB 2189545) that the spark surfaces should be made to resemble rails with inclined side surfaces sloping away from the spark surfaces and that the spark surface of the second electrode should be inclined relative to the aforementioned plane so that the spark gap varies in width with the narrowest point being nearest to the connection between the shell and the second electrode. This is said to cause the spark to be initiated at the narrowest point and advance along the gap in the direction in which it widens.
In the conventional spark plug described above the spark may cross the gap at any point along the length or across the width of the gap with the result that in some cases the spark is to some extent “masked” from the fuel mixture by the connection between the second electrode and the shell, thereby reducing the reliability of the ignition. It is an object of the present invention to overcome this disadvantage.